Hip replacement implants typically feature a stem with a head that cooperates with an acetabular cup. Hip stems are increasingly being provided in different sizes, lengths, and shapes. Some stems are also being provided with modular sleeves (also referred to as proximal bodies) that enable the stem to effectively “sit” in place with respect to the proximal femur. Sleeves or bodies in different sizes are provided to accommodate different bone structures and quality. The sleeves traditionally have a cone shape with a triangular spout extending from the cone, an example of which is shown by FIG. 7. The spouts approximate a portion of the proximal femur and provide additional support for the stem.
Preparing the proximal femur to receive a sleeve having a spout presents a challenge because bone must be removed in the shape of a triangle to receive the spout. In other words, once the distal femur has been reamed, a generally triangular shaped area needs to be milled out of the proximal reamer to receive the sleeve and spout. The surgeon should remove enough bone to achieve a secure fit, but not so much bone that the spout subsides and fails to support the stem as desired.
One previous preparation method has included aligning a shaft in the femoral canal and angling a cutter with respect to the shaft and moving the entire shaft within the canal to prepare the bone. An example of such a method is shown by FIG. 10 (which is reproduced from U.S. Pat. No. 5,002,578).
Another method has included aligning a shaft having an angled bearing in the femoral canal. A drill is inserted through the bearing at an angle to prepare a triangular cavity. An example of such a method is shown by FIG. 11 (which is reproduced from U.S. Pat. No. 5,540,694).
A variety of problems are encountered when using the methods and instrumentation of these procedures. For example, inserting a shaft into the canal and then separately inserting a drill through a bearing of the shaft causes the surgeon to have to maneuver multiple parts while also having to pay strict attention to the angles involved. One reason this causes a challenge is because the surgeon is holding the shaft at one angle (e.g., in the axis of the femoral canal) and maneuvering the drill at another angle (e.g., at an angle to form a triangular cut with respect to the axis of the canal), all while having to control the depth of the drilling. The surgeon often needs to drill the bone, remove the drill to check depth and shape of the cavity, and then reinsert the drill and continue the preparation. Although surgeons have become quite adept at these procedures, there is still a great deal of guess work involved. If too much bone is removed, the surgeon will often be forced to move to the next largest size of sleeve to accommodate for the excess bone removed.
Accordingly, it is desirable to provide more accurate milling methods that provide a precise cut. It is also desirable to provide a milling system that allows the surgeon to mill in a single direction, without having to enter the patient's leg at two different angles. (This is also beneficial to the patient because it is less invasive and a smaller incision can be used.) It is further desirable to provide a milling system that can be pre-assembled (e.g., on the back table by a nurse while the surgeon is preparing the site), which enables the milling to take place in one step. The systems and methods described herein provide many of these solutions.